1. Field of the Invention
The present invention relates to the protection of components connected to a supply line and liable to be damaged by overvoltages occurring in the supply line and, more particularly, to a circuit for protecting such components.
2. Discussion of the Related Art
The present invention is particularly useful for the case when the component is a circuit comprising active elements liable to be damaged by overvoltage, this component exhibiting between its terminals an essentially resistive impedance R that normally conducts a d.c. current having a relatively high value therethrough. This component can, for example, be a ballast lamp including an internal converter powered by a d.c. voltage. This component will be referred to hereinafter as a load to emphasize the fact it is a component that normally conducts a relatively high current therethrough.
Conventional protection circuits are generally divided into two types: series protection circuits and parallel protection circuits.
A series protection circuit is schematically shown in FIG. 1. The circuit is disposed between terminals A and B of a line and terminals C and D of a load L. The following addresses the case when the voltage between terminal A and B is a d.c. or rectified a.c. voltage, terminal A being the high voltage terminal. The series protection circuit comprises, between terminals A and B, a detection circuit 1 disposed so as to detect the occurrence of overvoltage and, between terminals A and C (or B and D), a switch S1, normally on, which is switched off when the detection circuit 1 detects an overvoltage.
FIG. 2 shows a parallel protection circuit which comprises a switch S2 disposed between terminals C and D. Switch S2 is normally off, and is switched on in response to the detection of an overvoltage by circuit 1.
Numerous embodiments of series and parallel protection circuits are known in the art, and will not be described in detail herein. However, it is to be noted that, in numerous practical implementations, a single circuit component performs both the switching and detection operations. In addition, in parallel protection circuits, the switch can act either really as a switch, as represented in FIG. 2, or as a clipping component allowing passage of overvoltages over a predetermined voltage threshold. Prior art circuits are known to simultaneously use series and parallel protection components, one of which is, for example, designed to operate in case of overvoltages occurring with an abrupt rising edge, and the other in case of overvoltages with a slower rising edge.
In practice, in the past, parallel protection circuits have been predominantly used. However, the drawback of these circuits is that, when it is desired to eliminate relatively high overvoltages, the circuits must be capable of withstanding high overcharges. Hence, it is necessary to provide large-scale semiconductor components capable of withstanding high thermal shocks. Thus, the present trend is to use, as much as possible, series protection circuits.
Series protection circuits, however, also exhibit drawbacks. In particular, with reference to FIG. 1, the abrupt switching off of switch S1 in response to the detection of an overvoltage is liable to cause a still higher overvoltage back in the mains, this overvoltage being equal to l.dI/dt, where l is the line inductance. This drawback particularly occurs if the rated current in the load is high.